Antigen Presentation and Autoimmune Disease

 Paul Lythgo

MHC class II antigen presentation is the process whereby exogenous proteins are degraded, loaded onto an MHC class II molecule, and presented on the cell surface to CD4+ T-cells. Before this peptide-MHC complex can form, proteases of the endocytic pathway must degrade the invariant chain (Ii), a protein which blocks the MHC class II binding site. This degradation occurs in a stepwise fashion:


The initial cleavage of the full-length Ii is carried out by an unknown aspartyl protease. Using a B-lymphoblastoid cell line (“Pala”) which overexpresses Ii, my research aims to characterize this initial cleavage. So far, I have screened Pala cells for the expression of all known human aspartyl proteases. Using RNA interference (RNAi), I am currently attempting to reduce expression of these proteases. This ablation is quantified with qRT-PCR, and the functional effects are visualized using Western blots.


Identifying the protease involved in this initial cleavage will have profound clinical implications. Our lab has previously shown that current HIV protease inhibitors (Ritonavir, Indinavir, Nelfinavir) can inhibit this initial Ii cleavage step with varying efficiency. Impairment of this step is intuitively detrimental to normal immune function. Currently, we are developing an immunoprecipitation assay aimed at detecting Ii in healthy white blood cells. In collaboration with Dr. Helene Cote, we will attempt to use this assay to detect levels of Ii in white blood cells derived from HIV-infected patients undergoing various protease inhibitor treatment.